The correct option is
C Hormonal control of sex
This condition causes infertility in the female cattle born twin to a male. When a heifer (a cow that has not borne a calf, or has borne only one calf) twin shares the uterus with a bull fetus, they also share the placental membranes connecting the fetuses with the dam. A joining of the placental membranes occurs at about the fortieth day of pregnancy, and thereafter, the fluids of the two fetuses are mixed. This causes exchange of blood and antigens carrying characteristics that are unique to each heifers and bulls. When these antigens mix, they affect each other in a way that causes each to develop with some characteristics of the other sex. Although the male twin in this case is only affected by reduced fertility, in over ninety percent of the cases, the female twin is completely infertile. Because of a transfer of hormones or a transfer of cells, the heifer's reproductive tract is severely underdeveloped and sometimes even contains some elements of a bull's reproductive tract. A freemartin is genetically female, but has many characteristics of a male. The ovaries of the freemartin do not develop correctly, and they remain very small.
So, the correct answer is 'Hormonal control of sex'.